This invention relates to a high impedance (Hi-Z) wire that is effectively transparent to radiation polarized in the direction of the wire, within an operating frequency band. The wire is sheathed with a thin layer of resonant structures, forming a photonic band gap (PBG) material. Out of band frequencies are reflected by the wire, frequencies within the operating band are unaffected. Such wires are more physically rigid than dielectrics and can be applied to non-interactive antenna support stays, dispersive polarizing beam splitters, or wire grid reflectors for focusing radiative power.
The assembly of PBG materials has recently been advanced at UCLA (University of California at Los Angeles) using printed circuit board techniques to make a two dimensional array of sub-wavelength scale resonant structures on the surface of the board. These concepts are referred to in U.S. patent application Ser. No. 09/537,923 entitled xe2x80x9cA Tunable Impedance Surfacexe2x80x9d filed on Mar. 29, 2000 and U.S. patent application Ser. No. 09/525,255 entitled xe2x80x9cRadio Frequency Aperturexe2x80x9d filed on Mar. 14, 2000.
A conventional high-impedance surface, shown in FIG. 1, consists of an array of metal top plates or elements 13 on a flat metal sheet 12. It can be fabricated using printed circuit board technology with the metal plates or elements 13 formed on a top or first surface of a printed circuit board and a solid conducting ground or back plane 12 formed on a bottom or second surface of the printed circuit board. Vertical connections are formed as metal plated vias 14 in the printed circuit board, which connect the elements 13 with the underlying ground plane 12. The metal members, comprising the top plates 13 and the vias 14, are arranged in a two-dimensional lattice of cells, and can be visualized as mushroom-shaped or thumbtack-shaped members protruding from the flat metal surface 12. The top plates or elements 13 are preferably hexagonal and the thickness of the structure, which is controlled by the thickness of the printed circuit board, is much less than one wavelength for the frequencies of interest. The sizes of the elements 13 are also kept less than one wavelength for the frequencies of interest. The printed circuit board is not shown for ease of illustration.
Turning to FIG. 2, the properties of this surface can be explained using an effective circuit model which is assigned a surface impedance equal to that of a parallel resonant LC circuit. The use of lumped circuit elements to describe electromagnetic structures is valid when the wavelength is much longer than the size of the individual features, as is the case here. When an electromagnetic wave interacts with the surface of FIG. 1, it causes charges to build up on the ends of the top metal plates 13. This process can be described as governed by an effective capacitance C. As the charges slosh back and forth, in response to a radio-frequency field, they flow around a long path P through the vias 14 and the bottom metal surface 12. Associated with these currents is a magnetic field, and thus an inductance L. The capacitance C is controlled by the proximity of the adjacent metal plates 13 while the inductance L is controlled by the thickness of the structure. The structure is inductive below the resonance and capacitive above resonance. Near the resonance frequency       ω    =          1              LC              ,
the structure exhibits high electromagnetic surface impedance. The tangential electric field at the surface is finite, while the tangential magnetic field is zero. Thus, electromagnetic waves are reflected without the phase reversal that occurs on a flat metal sheet. In general, the reflection phase can be 0, xcfx80, or anything in between, depending on the relationship between the test frequency and the resonance frequency of the structure. The reflection phase as a function of frequency, calculated using the effective medium model, is shown in FIG. 3. Far below resonance, it behaves like an ordinary metal surface, and reflects with a xcfx80 phase shift. Near resonance, where the surface impedance is high, the reflection phase crosses through zero. At higher frequencies, the phase approaches xe2x88x92xcfx80. The calculations are supported by the measured reflection phase, shown for an example structure in FIG. 4.
It would be useful for numerous applications if it were possible to cover or coat a wire with a Hi-Z surface, so that the wire would behave like a Hi-Z structure. However, the structure of the Hi-Z surface, as described in the prior art, does not lend itself to such covering or coating of a wire. The present invention overcomes this difficulty and provides techniques for disposing Hi-Z surfaces on wires. A technique for electrically isolating a wire by modifying its behavior from a low resistance short to a highly reactive current path is provided.
Metal guy wires, stays or struts are often the preferred construction technique for stiffening mountings and long posts; or for suspending objects away from walls or ceilings. For microwave applications, for example for mounting a detector horn at the focus of a parabolic reflector, metal parts can be added that will not interfere with the desired propagation of the electromagnetic signal. The supports no longer need to be a source of interference.
The prior art includes RF reflector and focal plane sensor systems. Typically, satellite antennas deploy a detector at the focus of an offset parabolic reflector, such as with DirecTV(trademark) or DirecPC(copyright). The parabola is offset for reasons that involve beam blockage and diffraction by the supports. This invention enables other construction techniques with better overall performance.
Baluns (typically ferrite beads with high magnetic permeability or balun transformer cores) are sometimes slipped over a wire to induce a high inductive reactance for a lead. In effect it is a low pass filter. High frequencies are reflected or absorbed by losses in the balun. Thus, generally the balun""s effect is used for blocking out of a band noise. The present invention has low loss and the frequency of operation is more controllable than that which can be achieved with magnetic materials.
The Hi-Z wire of the present invention can be applied to microwave polarizers. One conventional method of producing a microwave polarizer is to use a layer of thin wires spaced less than a wavelength apart and aligned in the same direction, thereby forming a grid. An incoming electromagnetic wave will have its electric field component parallel to the wires reflected, and its component orthogonal to the wires undeflected by the grid. When Hi-Z wires (i.e., covered with a PBG medium) are used in the grid, the polarization effect is frequency dependent, which makes the polarizer band selective. This feature provides a useful improvement over conventional microwave polarizers.
Hi-Z wires can be used to construct a Low/Hi-Z Fresnel reflector which improves on traditional Fresnel reflectors. By using an array of wires with spacing on the order of xc2xd wavelength, one can reflect a wave to various angles similar to a conventional grating. However, this configuration has low efficiency due to the wide spacing of the wires. By placing Hi-Z wires between the ordinary wires, the efficiency is significantly improved. This is only possible with Hi-Z wires.
In accordance with this invention, a metal wire is sheathed with a thin layer of resonant structures, forming a Hi-Z (high impedance) wire that is effectively transparent to radiation polarized in the direction of the wire within an operating frequency band. These structures are small compared to a wavelength and can be fabricated in mass production. Since the wire sheathing is effectively a photonic band gap layer, out of band frequencies will be reflected by the wire. Hi-Z wires are more rigid than dielectrics, and can be applied to non-interacting antenna support stays.
In another aspect of this invention, Hi-Z wires are disposed parallel to one another in a grid to form a frequency-selective microwave polarizer. Outside a certain frequency band, the electric field component parallel to the wires is reflected by the polarizer, whereas the orthogonal component passes through unaffected. Within a certain frequency band, the wires appear transparent to the radiation and no polarization occurs. The polarizing effect is thus frequency selective.
In yet another aspect of the invention, Hi-Z wires are interspersed with conventional wires and disposed in a grid to form a Fresnel reflector. This configuration enables stepwise phase control of the reflected phase.
In yet another aspect of this invention, a method of sheathing a wire with a thin layer of resonant structures is provided, as well as a method of polarizing electromagnetic radiation.